This invention relates to a shadow mask having holes arranged with a varying, not constant, arrangement pitch for use in a color cathode-ray tube, and to a color cathode-ray tube having such a shadow mask.
In general, a color cathode-ray tube for displaying a color video image is formed of a panel portion with a phosphor screen formed on a inner surface of a faceplate, a neck portion in which an electron gun is provided for generating three electron beams, and a funnel portion for connecting the panel portion to the neck portion. The funnel portion has provided thereon a deflector for deflecting the electron beams emitted from the electron gun, causing the beams to scan the phosphor screen.
The electron gun has various electrodes such as cathode electrodes, control electrodes, focus electrodes and accelerating electrodes. Three electron beams emitted from the cathode electrodes are modulated by image signals which are supplied to the control electrodes, and then passed through the focus electrodes and accelerating electrodes, thereby having their energy increased. The electron beams impinge on the phosphor screen. During the period in which the three electron beams are emitted from the electron gun and arrive at the phosphor screen, they are horizontally and vertically deflected by the deflector which is provided on the funnel portion, and then, as is well known, pass through holes in a shadow mask to produce a color video image on the phosphor screen which is provided just after the shadow mask. In this case, the shadow mask is formed in a curved shape which matches a curved shape (a slightly spherical shape) of the faceplate of the panel portion, and the arrangement pitch of the holes in the shadow mask is determined in accordance with this curved surface.
FIGS. 1a and 1b are schematic diagrams to which reference is made to explain how to determine the horizontal arrangement pitch of the holes in the shadow mask. FIG. 1a is a cross-sectional diagram of part of the inside of the panel portion of a color cathode-ray tube, and FIG. 1b is a plan view of part of the shadow mask.
In FIGS. 1a and 1b, 50 denotes a faceplate of the panel portion, 51 a shadow mask, 52 a central axis of the tube, 53 an electron beam, 54 a center of deflection of the electron beam 53, 55 slit-shaped electron beam passage holes, and 56.sub.0, 56.sub.1, 56.sub.2, 56.sub.3 . . . 56.sub.n-1, 56.sub.n. 56.sub.n+1, 56.sub.n+2 the vertical center lines connecting respective electron beam passage holes 55.
Within the panel portion of the cathode-ray tube, the shadow mask 51 formed in a slightly spherical curved shape is disposed, in substantially parallel to and opposing, faceplate 50 which is also formed similarly in a slightly spherical curved shape. The electron beam 53 emitted from the electron gun (not shown) and traveling along the tube axis 52 is, for example, horizontally deflected, at the deflection center 54, and then projected through the shadow mask 51 to the phosphor screen (not shown) which is provided on the faceplate 50.
Here, the distance between the shadow mask 51 and the faceplate 50 along the path of the electron beam 53 is represented by b.sub.n, the distance between the deflection center 54 and the faceplate 50 by d.sub.n, the pitch (hole arrangement pitch) of the vertical center lines 56.sub.n-1, 56.sub.n by a.sub.n, and a constant to be determined by the deflector (not shown) by S. Then, the distance b.sub.n can be expressed by the equation of b.sub.n=a.sub.n.multidot.d.sub.n/ 3S, or satisfies the relation of b.sub.n .alpha.a.sub.n. Thus, the distance b.sub.n is determined over the entire surface of the shadow mask 51, and the shape of the shadow mask 51 is determined so that this distance b.sub.n is smoothly or continuously changed over the entire surface, and the hole arrangement pitch a.sub.n at each portion is determined in accordance with the shape of the shadow mask.
In this case, the shape of the known shadow mask 51 is designed so that the radius of curvature at the periphery (represented by R2) is somewhat smaller than that at the center (represented by R1), or that R1&gt;R2. Thus, the pitch of a large number of electron beam passage holes provided in the shadow mask 51 is gradually increased from the center toward the periphery.
The shadow mask 51 is generally made of a thin iron plate about 0.1 to 0.3 mm thick, and is shaped so that it is slightly spherical as described above. Since the whole mask is made of a thin iron plate, its strength is not enough, and thus this mask has various known problems as described below.
When the electron beam 53 impinges on the shadow mask, the shadow mask 51 thermally expands due to heat generated by the impingement. As a result, the trajectory of the electron beam 53 is changed, causing purity degradation, namely, a so-called "doming" occurs Further, the shadow mask 51 may be resonant to sound (particularly from a loud speaker) generated by the television set or a vibration, causing purity degradation, or a so-called howling. In addition, the shadow mask 51 may be deformed by a shocks occuring during a manufacturing process for the color cathode-ray tube or transportation thereof, thus causing purity degradation.
A color cathode-ray tube in which the shadow mask is designed so that the distance between adjacent columns of holes in the shadow mask is the greatest at its central and end portions and is the smallest midway between the central and end portion is disclosed in JP-A-59-165341(published on Sep. 18, 1984).